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THE EINSTEIN-GROSSMANN COLLABORATION 219

From this tensor 'it is ... possible to derive a second-rank tensor of the second

order [in the derivatives of £„,],' the Ricci tensor:

"The specifications of the actual rods and clocks suited for this purpose are delicate [Ml]. This

author must confess to an occasional doubt as to whether this problem has as yet been fully under-

stood on the atomic and subatomic levels.

Having come this close, Grossmann next makes a mistake to which I shall return

in the course of describing Einstein's contributions to EG, a topic which should

be prefaced by stating Grossmann's agreement with Einstein. 'With pleasure, he

[G.] was ready to collaborate on this problem under the condition, however, that

he would not have to assume any responsibility for any assertions or interpreta-

tions of a physical nature' [E32].

Einstein begins by stating his desideratum: to generalize the theory of relativity

in such a way that his earlier result on the variability of the light velocity in an

inhomogeneous static gravitational field [E33] shall be contained as a special case.

Without preliminaries, he turns at once to the demand of general covariance: the

motion of a mass point shall be determined by Eqs. 11.12 and 12.1, which I copy:

These equations shall be invariant under the transformations (Eqs. 12.9 and

12.10), and ds^2 shall be an 'absolute invariant.' Then he goes on to state the prin-

ciple of equivalence as we know it today: there is a special transformation of the

type(Eq. 12.9):

that brings the quadratic form (Eq. 12.22) locally on principal axes:

This local coordinate frame in which the gravitational field has been transformed
away acts as a free-falling infinitesimal laboratory. Time and space measurements
can be performed locally in this frame by the same methods used in the special
theory of relativity.* It follows that in terms of the general dx^1 ', as in Eq. 12.22,
'the corresponding natural distance can be determined only when the g^ which
determine the gravitational field are known.. .. The gravitational field influences
the measuring bodies... in a definite way.' With these words, he states the broad
program of the general theory of relativity.